Growing quality, energy and environmental concerns have produced a drive to simultaneously reduce solvent emissions in coatings; improve coating performance, e.g., by post-crosslinking coating polymers; reduce toxicity; and reduce cure temperatures. In order to reduce solvent emissions, it is possible to employ aqueous or powder coatings, but this is usually accomplished at a high cost in performance, coating appearance, high cure temperature and storage stability or pot life.
Alternatively, solvent-borne coatings are widely used, but prepared at high solids to minimize solvent emissions and low molecular weight to give usable viscosities for spray or brush application. To provide good coating performance, e.g., solvent and water resistance, hardness, toughness, scratch resistance and the like, it is necessary to crosslink or cure the coating polymer after application to build a high molecular weight. Many technologies are known for doing this, but most suffer from one or more drawbacks, including slow and inefficient cure, high toxicity of reactive cure components, incompatibility with water, air or other coating components, high cost, low durability or poor storage or pot stability before coating application.
The most widely applicable functionality for achieving practical solutions to the problems of stability, high reactivity with a wide variety of crosslinking functionalities, low toxicity and efficient crosslinking is the primary amine group attached to an appropriate hydrophobic polymer or oligomer backbone. Such functionality is reactive with epoxides, isocyanates, amide/formaldehyde and other aldehyde condensates (aminoplasts), Michael acceptors, aziridines, acetylacetates, anhydrides, lactones and other active esters, ketenes and ketene dimers, aldehydes and ketones, coordinating transition metals, alkylating agents (or their polymeric equivalents) and acid halides, to name the more common reaction partners. Unfortunately, there are very few ways to prepare primary amine functional polymers and oligomers, especially using low cost, free-radically polymerizable monomers without introducing solvent sensitive or hydrolytically unstable functionalities to link the primary amine group to the polymer chain.
Amine functional low polymers and oligomers have been prepared by condensation polymerization of di- or higher amines with diacids or esters, diisocyanates, di- or higher functional Michael acceptors (e.g., ethylene glycol diacrylate (EGDA)), reduction of diolacrylonitrile adducts, or reductive amination of diols. These approaches are frequently limited to di- or lower amine functionality, are restricted in molecular weight and attainable T.sub.g and, as in the case of aminated diols, have hydrophilic backbones. Typical ethylenediamine-based products also have poor outdoor weatherability. Condensation reactions are also frequently difficult to control and give a broad molecular weight distribution, color, and in some systems, such as those based on isocyanates, are quite expensive. Many condensation-based di- and polyamines are found predominately only as high amine functional low molecular weight curatives.
Addition of explosive, carcinogenic and highly toxic aziridines to di- or polycarboxylic acids to produce amine functionality is known, but the real and perceived manufacturing difficulties and hazards of producing the products have kept these materials from wide acceptance.
The highly desirable option of preparing high performance polyamine functional polymers and oligomers via low cost free radical copolymerization of widely available vinyl monomers with an amine functional vinyl monomer has been severely constrained by the lack of a decently copolymerizable amine monomer. U.S. Pat. Nos. 4,504,640 and 5,155,167 use allyl and diallyl amines for this purpose, but these monomers are well-known to undergo severe chain transfer reactions and lead to serious polymerization inhibition, due to the well documented tendency of alkylamines, and especially allylamines, to lose a hydrogen atom alpha to the nitrogen. This tendency is somewhat suppressed in methacryloxyethyldiakylamine or their salts, but these monomers lack a reactive amine hydrogen for the subsequent crosslinking reaction. t-Butylaminoethyl methacrylate has also been proposed for this application, but contains only a highly hindered secondary nitrogen with poor reactivity and the monomer is expensive and unstable.
These deficiencies can, in principle, be alleviated by copolymerizing a protected vinyl functional amine monomer to appropriate co- and terpolymers and subsequently removing the blocking group. Vinylamides, imides and carbamates have been frequently proposed as polymerizable precursors to the attractive, but chemically unstable and unavailable `vinylamine` monomer. Of these, higher amides, imides and N-vinylpyrrolidone are notoriously hard to hydrolyze under realistic conditions and success has frequently been achieved using toxic hydrazines under commercially unrealistic conditions. More recently, the use of N-vinyl-O-t-alkyl carbamates or N-vinylformamides has been proposed and demonstrated to allow hydrolysis to the amine functionality under commercially reasonable acid or (for the formamides) base conditions. Attempts to reduce the above concept to practice, however, resulted in severe difficulties. Co- and terpolymers of N-vinylformamide (NVF) are readily prepared with acrylates and, using appropriate monomer delay procedures, with methacrylates. However, on attempted deblocking of the amine group, a very rapid reaction occurs with neighboring ester groups to give a thermodynamically and kinetically favored .gamma.-lactam with poor or no reactivity with most amine reactive functionalities. Recourse to styrene and related comonomers is an obvious next step, but NVF undergoes slow, inefficient and incomplete polymerization with styrene, mirroring the notoriously poor polymerizability of its close analog, vinyl acetate (VAc) with styrenics. A similar fate would seem likely using butadiene and its analogs. Use of ethylene (or, less advantageously, higher olefins) as the predominant comonomers is a possibility, but the requirement for extremely high pressures, 15,000 to 25,000 psi, is a strong disincentive for most manufacturers.
U.S. Pat. Nos. 4,774,285, 4,880,497 and 4,978,427 disclose the use of vinyl acetate (VAc) and vinyl propionate (VPr) copolymers of protected vinylamine monomers, including N-vinyl formamide. It is suggested that these polymers hydrolyze under acid to give hydrophobic amine functional polymers, however, it is taught in the examples that these systems undergo rapid ester hydrolysis under acid or base to give hydrophilic, water soluble amine functional PVOH. It is reported specifically in the above patents that acid hydrolysis in water provides extensive formamide and acetate hydrolysis to fully water soluble polymers with low residual PVAc or PNVF functionality. Although the above patents correctly predict some selectivity for formamide over VAc or VPr under acid hydrolysis to give ammonium functional vinyl ester copolymer, rapid intramolecular reaction of amine groups with neighboring ester groups in acrylate/vinylamines would lead one skilled in the art to anticipate an analogous rapid reaction with adjacent ester groups in vinyl ester copolymers to give vinylamide/vinyl alcohol functionality of little utility for high performance coatings.
U.S. Pat. No. 3,558,581 discloses poly-N-vinyl-N-methylamine and copolymers of N-vinyl-N-methylamine with compounds polymerizable under the action of a free-radical liberating catalyst. The polymers are formed by synthesizing the corresponding N-vinyl-N-methylformamide and subsequently hydrolyzing with mineral acids. U.S. Pat. No. 5,064,9009 also disclosed vinylamide copolymers which are hydrolyzed and subsequently used in papermaking applications.